1. Field of the Invention
The present invention relates to a multi-phase phase-shift keying (PSK) modulation apparatus, particularly a four-phase PSK modulation apparatus which can be used in a wide frequency range, for example, 70 MHz.+-.18 MHz, 140 MHz.+-.36 MHz, etc., without or with reduced errors.
2. Description of the Prior Art
The attached FIG. 1 shows a block diagram of a four-phase PSK (4-PSK) modulation apparatus of a prior art; wherein 1 indicates a phase-locked oscillator of a fixed frequency F.sub.1, 2 indicates a 90.degree. hybrid providing 0.degree. and 90.degree. phase shifted signals, 3 indicates a 4-PSK modulator, 4 indicates an oscillator as a channel synthesizer and 5 indicates a mixer for mixing the outputs of the 4-PSK modulator 3 and the outputs of the channel synthesizer 4.
As it is clear from the constitution shown in the FIG. 1, the 4-PSK modulator 3 modulates the fixed frequency F.sub.1 by the baseband signals. The outputs of the 4-PSK modulator are then mixed down with the outputs of the channel synthesizer 4 of the frequency F.sub.2 .+-.BW by the mixer 5 to give outputs of the frequency F.sub.0 .+-.BW (wherein F.sub.0 =F.sub.2 -F.sub.1). In order to enable correction of any errors such as the amplitude error (.DELTA.AM), the orthogonality error (.DELTA..phi.) and the offset error (x), (y) as illustrated in FIG. 2, it was required to make the 4-PSK modulation at a fixed frequency F.sub.1, because these errors become too large and complicated to adjust, if the 4-PSK modulation is made at the final frequency F.sub.0 .+-.BW in a wider range.
For this reason, the prior art 4-PSK modulation apparatus had to employ a system of mixing down the 4-PSK modulated and corrected outputs of the fixed-frequency oscillator with the outputs of the channel synthesizer. Hence it had problems of (a) high spurious outputs, (b) need of two oscillators and (c) complicated constitution causing a large volume and a high cost.